The Master Slider – Responsive Touch Slider plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the plugins ms_slide shortcode in all versions up to, and including, 3.9.9 due to insufficient input sanitization and output escaping on user supplied css_class attribute. This makes it possible for authenticated attackers with contributor-level and above permissions to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.